Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.
Waste gas emissions are of significant concern, and the presence of certain gaseous constituents in a waste gas stream can result in air pollution. There is significant research into methods for treating waste gas streams to remove these gaseous constituents from waste gas streams. Carbon dioxide (CO2) emissions, in particular, attract a great deal of attention and the discussion of waste gas emissions that follows will largely be in respect of carbon dioxide. However, the skilled addressee will appreciate that much of this discussion is also applicable to other waste gases.
Emissions of CO2 from combustion processes are recognised as the single biggest contributor to the problem of excessive greenhouse gas concentrations in the atmosphere. One method of reducing atmospheric CO2 emissions is through its capture and subsequent storage in geological or deep sea reservoirs. Capture can be done either from a concentrated point source, such as a fossil fuel power station gas stream (post-combustion capture), or from the atmosphere directly (air capture).
The process for capturing CO2 is termed carbon capture and storage (CCS). In CCS, the CO2 is first separated from a gas mixture typically containing nitrogen, oxygen and possibly other gases using a suitable solvent in a gas-liquid contactor. The contactor may be a packed column, membrane, bubble column or other suitable device. The CO2 is then removed from the solvent in a regeneration process producing pure CO2, thus allowing the solvent to be reused. The CO2 is then liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. The liquefied CO2 is transported to a storage site such as a depleted oil or gas reservoir or deep saline aquifer where it is injected for geological storage.
CO2 is an acid gas. That is, upon dissolution in water it forms an acid (carbonic acid). As a consequence typical solvents for the reversible capture and release of CO2 are solutions of weak bases such as amines and/or carbonates. Weak bases allow absorbed CO2 to be released by heating due to a combination of reduced gas solubility and reduced basicity at elevated temperature. Strong bases such as hydroxide are superior in terms of CO2 absorption, however they do not allow for release of the absorbed CO2.
Aqueous amine solutions and alkanolamine solutions in particular, have been investigated extensively as solvents for CO2 capture. As well as being weak bases many amines also have the favourable property of reacting rapidly with CO2. Once dissolved into the amine solution, the aqueous CO2 reacts with water and the neutral form of the amine react to generate carbonic acid (H2CO3), aqueous bicarbonate (HCO3−) ions and aqueous carbonate (CO32−) ions, according to the generally acknowledged equations described below:CO2+2H2OH2CO3   (equation 1)CO2+OH−HCO3−  (equation 2)CO32−+H3O+HCO3−+H2O   (equation 3)HCO3−+H3O+H2CO3+H2O   (equation 4)OH−+H3O+2H2O   (equation 5)R1R2R3N+H3O+R1R2R3NH++H2O   (equation 6)
If the amine contains a primary (R1R2NH, R2═H) or secondary amine (R1R2NH, R2≠H), an additional reaction pathway becomes available, where carbon dioxide and the primary or secondary amine react to generate carbamic acid (R1R2NCOOH) and its conjugate base carbamate (R1R2NCOO−). Typically the carbamic acid is a stronger acid than its amine precursor and it exists predominantly in its carbamate form. The affinity of a primary or secondary amine to react with CO2 to form carbamate is governed by the electronic environment and the level of steric crowding of the amine functional group. The carbamate may also then participate in acid-base chemistry, according to the generally acknowledged reactions described below. Tertiary amines (R1R2R3N, R1, R2, R3≠H) cannot form carbamates.CO2+R1R2NHR1R2NCOOH   (equation 7)R1R2NCOO−+H3O+R1R2NCOOH+H2O   (equation 8)
It is generally acknowledged that the molar absorption capacity of an aqueous amine solution, as measured by the number of moles of CO2 absorbed per mole of amine functionality in solution (α), is dependent upon the pH equilibria that operate in the amine solution and the formation of carbamate species.
CO2 desorption is normally achieved by heating of an aqueous amine solution containing CO2. The two major effects of heating are to reduce the physical solubility of CO2 in the solution, and to reduce the pKa of the amine resulting in a concomitant reduction in pH and in CO2 absorption capacity, the net effect of which is CO2 release. The extent of the reduction in pKa is governed by the enthalpy of the amine protonation reaction which in turn is governed by the amine chemical structure. All the other reactions, including carbamate formation, have small reaction enthalpies and are as a result relatively insensitive to temperature. Typically, the enthalpy of amine protonation is four to eight times larger than the enthalpies of the carbonate reactions and two to four times larger than the enthalpy of carbamate formation. It is predominantly the lowering of the pH upon heating that drives the reversal of carbamate and carbonate/bicarbonate formation during desorption, rather than any significant reduction in stability.
In summary, there exists a relationship between the change in amine pKa as a function of temperature and the concomitant lowering of the solution pH, and an aqueous amine solutions ability to absorb and release CO2.
The thermal energy requirement for the release of CO2 is considerable. For CO2 capture from a relatively concentrated CO2 source such as a coal fired power station some 3-4 GJ is required per tonne of CO2 captured. This consumes 20-30% of a power stations energy output. Even more energy is required per unit of CO2 captured from a dilute source such as the atmosphere. This means air capture using such technology is only feasible in close proximity to a large energy source, but is generally energetically unfavourable.
Identification of the problem of the large energy requirement for release of CO2 from a solvent has lead to investigation into ways this can be reduced. One approach is through solvents which have an increased capacity to cyclically absorb and release CO2. However, amines used for industrial CO2 capture that achieve larger CO2 cyclic absorption capacity typically have poor rates of CO2 absorption. Slow CO2 absorption rates are undesirable because to achieve the requisite absorption of CO2 longer gas-liquid contact times are required which means larger absorption columns and greater capital cost. The benefits gained through increased cyclic capacity are thus offset by the disadvantages associated with decreased rates. Furthermore the improvements in terms of reduced energy demand are incremental at best and do not reduce it to the point of making CO2 capture possible without energy penalty.
Many of the issues discussed above in relation to CO2 capture apply to a range of other gaseous pollutants. It is therefore pertinent to identify solvents or solutions with significantly reduced gas release energy requirements for application in gas capture technologies.